A Channel Quality Indicator (CQI) is needed for link adaptation in wireless communication systems. The CQI should reflect the transmission capacity of a frequency-time channel. Thus in wideband systems with frequency selective channel fading, for example Long-Term Evolution (LTE), LTE-Advanced, and Worldwide Interoperability for Microwave Access (WiMAX), the CQI can be either of wideband type to cover the whole frequency bandwidth, or it can be of frequency selective type, which means that each CQI only covers a part of the bandwidth. For frequency selective CQI, the finer frequency granularity will lead to better channel dependent scheduling and more accurate link adaptation, thus result in higher throughput, lower BLER and shorter packet transmission delay.
However, fine frequency granularity will cause a big feedback overhead for CQI report, since the number of CQIs to cover the whole bandwidth is large. CQI compression methods are employed to save signaling overhead. In LTE, a compression method called UE-selected or Best-M is employed.
By this way, the whole bandwidth is divided into several subbands, and only a WideBand (WB) CQI and M CQIs for the M Subbands (SBs) with best Signal to Interference and Noise Ratio (SINR) are reported.
Besides frequency granularity, the age of CQI also impacts the accuracy of link adaptation, and degrades the throughput. The younger age leads to better accuracy.
In LTE, UE-selected CQI report behavior is decided by the feedback channel type. CQI can be reported in two feedback channels: physical uplink control channel (PUCCH) and physical uplink shared channel (PUSCH). PUCCH is periodic, and it does not need a scheduling trigger. PUSCH CQI is aperiodic, and it needs the signaling from the base station to trigger the report.
For PUCCH UE-selected CQI, the whole bandwidth is divided into M so-called bandwidth parts, and each bandwidth part contains several subbands. In each bandwidth part, one best subband CQI will be reported. WB CQI and SB CQI are reported in different time instance, and between two consecutive WB CQI report instances, the best SB CQI within each bandwidth part will be reported in turn, (from the 1st bandwidth part to the last one). When all the bandwidth parts have reported its best SB CQI, a new same cycle of SB CQI reports can be initiated, or a WB CQI is reported.
For LTE PUCCH UE-selected CQI, the base station can use both WB CQI and UE-selected SB CQIs for scheduling and link adaptation. It is intuitive to use UE-selected SB CQIs for the corresponding Best-M subbands, and use WB CQI for the remaining subbands.
This solution is simple and effective if the CQI is not out-dated. However CQI can be easily outdated in practical systems, and the performance of CQI will be severely degraded.
The reason for an out-dated CQI may be a long report period. Firstly, for each PUCCH UE-selected CQI report instance, only one WB or SB CQI can be reported, and it will take a long time to update the WB CQI and all SB CQIs, especially when there are several SB CQI cycles between two WB CQIs. Secondly, since PUCCH bandwidth is very limited, when there are many user equipments (UEs) in the system, the CQI report period has to be even longer to support the CQI report of all the UEs.
In case the UE moves at a high speed the channel varies fast, and is very easily out-dated. Even for a low speed, the wideband SINR variation can be still obvious between two consecutive WB CQI updates.
It can be concluded that the WB CQI of LTE PUCCH UE-selected CQI is very likely to be out-dated. Though the best SB CQI is always younger than the WB CQI, it can not be guaranteed that only these best subbands are scheduled. If the non-best subbands are scheduled, only out-dated WB CQI can be used for their link adaptation, and the accuracy will be bad and higher block error rate (BLER) and low throughput will be achieved.
This means that the benefits of reporting CQI with finer frequency granularity is canceled out by a longer WB CQI update period.
There are some techniques which can be employed to solve the out-dated CQI problem, but these cannot provide a satisfactory solution to the problem.
There are two major ways by which channel quality is predicted, one of which combines past and current channel quality measurements to predict future channel quality. Both WO2004052982 A2 and WO2008041893 A1 employ such a method. However, none of them take the Best-M subband or UE-selected subband CQI report into consideration for prediction of channel quality.
From WO 2004052982 A2 it is disclosed a method wherein past, current and predicted CQIs are derived from one and the same downlink channel. It is a disadvantage that the channel quality at the time instances of reporting these CQI values should be related to achieving an effective prediction. Moreover, the prediction process is carried out at the UE side, which is a further disadvantage since different UEs will use different prediction algorithms. The base station will then be unaware of the algorithms used and will treat all the predicted values the same way, independently of the algorithm used by each UE. Since some CQI values are predicted for some UEs but for not for others, this clearly results in a non optimized overall CQI prediction.
Moreover, in the case only the predicted CQI is reported, different UEs can employ different algorithms to perform prediction, which can lead to different behaviors among the UEs, in case different algorithms are used. In addition, in the case raw CQI and predicted CQI are reported simultaneously, the overhead is increased on the signaling channel, which can find support by few standards only, excluding LTE.
From WO2008041893 A1 it is disclosed a prediction method to be carried out by the base station. This method does not take the frequency domain into account, which is a disadvantage. Moreover, a prediction table is required that is built by using a significant amount of measured CQI instances. This table is based on a number of conditions, for instance busy hour and night time, user equipment types etcetera, which means that it is difficult to create in reality. Further more, when using it for LTE PUCCH UE-Selected CQI reports, the WB CQI report period can be very long, for which reason the consecutive WB CQIs reports are not related in the time domain which consequently makes the prediction less efficient.
Another way to perform channel quality prediction is to reduce CQI with a fixed margin to cover channel variation during a fixed period. For instance, if the maximum Signal to Interference and Noise Ratio (SINR) variation within 10 ms is 1 dB, the CQI could be reduced with 1 dB or more. The deficiency is that CQI values are reduced with no relation to whether they are out-dated or not. Such a uniform decrease clearly impairs the system which can lead to lower system throughput.
Still another way to perform channel quality prediction may be by using outer-loop CQI adjustments based on positive acknowledge (ACK) and negative acknowledge (NACK) feedback. However, this kind of outer-loop CQI adjustment uses one and the same adjustment step for all the subbands, which will impair the UE-selected SB CQI that are not out-dated. Another drawback of this alternative way is that the method is unable to adjust the CQI in advance of a transmission failure. Also, the time required for the adjustment is relatively long. Transmission in a system using this method for prediction of a channel quality is most likely to fail just before the next WB CQI update instance. At this time, the WB CQI is most out-dated, for which reason it can not provide a fast reliable prediction of CQI.
In addition, if a transmission from a UE to a base station fails just at the end and a NACK is reported to the base station, the base station will decrease the CQI. However, if the NACK is reported after the next WB CQI is reported, the base station will unnecessarily adjust the new WB CQI, which can also lead to lower throughput.
It should be noted that Channel Quality Indicator, CQI, in addition to Precode Matrix Indicator, PMI and Rank Indicator, are all comprised in Channel State Information, CSI. CQI is thus an example of CSI.
There is a need for an improved method and arrangement for CSI prediction that will provide adequate predicted values for different UEs when employing a CSI report pattern of wideband CSI, followed by several subband CSIs before another wideband CSI is reported.